Thermal-transfer recording methods using thermal heads have become widely used for a variety of utilities such as label printers, ticket venders, word processors etc. As the use has spread, the usage environment of the prints has become more severe than the environment under which they were conventionally used.
Further, as the usage environment of the prints has become more severe, types of transfer media used have diversified from paper as conventionally used to plastic films etc. which have less dependence on the environment. When the transfer material is atypical, conventional ribbons having a thermal-transfer ink composition mainly composed of waxes cannot provide satisfactory transfer of ink or the print tends to easily rub off due to a slight abrasion even after successful transfer, so that the print cannot provide satisfactory mechanical resistance to abrasion.
In particular, in the field of printed matter needing high-quality, posters, billboards etc., there are strict constraints on the reproducibility of colors and color unevenness of the transfer material, and no conventional thermal transfer ribbons can satisfy these requirements.
Further, concerning the conventional thermal transfer ribbons, in the case where color representation is made by repeated transfer operations of thermal-transfer ink layers onto the same transfer medium and when two or more repeated printings are made, the composition of the previously printed transfer ink layer will melt from heat from the thermal head when it makes a repeated printing operation because the viscosity of the conventional thermal transfer ink at the softening point or at the melting point is low. This melting causes color unevenness due to ink mixing and ink repellence, and deficiency in transfer itself in the worst case.
The countermeasures against these problems needed delicate mechanical and electrical controls as being effected by impregnating all the inks which are transferred multiple times into paper as transfer medium so as to produce a mixed state of inks for representation of colors, or by lowering the transfer energy on the printer side depending on the number of repetitions of transfer, i.e., the first, the second and the third, so as to maintain the transferability in a good state.
There has been another attempt in which in accordance with the number of repetitions of transfer and the order of transfer, the softening temperatures of the ink layers to be transferred are differentiated so as to attain both good transfer performance and reproducibility of colors.
These countermeasures are effective in the case where the transfer media is composed of a material such as paper and the like, but cannot exhibit satisfactory effects for materials being less absorptive such as a plastic base etc., during transfer of thermal transfer ink.
As stated above, the environment under which the thermally transferred prints produced by using thermal heads has become more severe than the environment under which the prints were conventionally used. Examples include use under severe room temperatures and use under an environment in which the prints are mechanically abraded.
Because of this tendency, special types of transfer media which are more hard-wearing have been used as already stated. This tendency requires good transfer to the plastic film etc., acquisition of printed matters having the required durability, and solving the problems of ink unevenness due to mixing of inks, repellence of ink, and weak fixing performance due to superimposition of inks when representation of colors is made by repeated transfer of thermal-transfer ink layers onto the same transfer medium.
Therefore, there is an important demand for a thermal-transfer recording medium meeting all the requirements such as being able to perform good transfer to transfer media having durability, such as plastic film, to provide prints having sufficient resistance to mechanical abrasion without causing color unevenness due to ink mixing or ink repellence when representation of colors is made by repeated transfer of thermal-transfer ink layers onto the same transfer medium, and to provide sufficient resistance to abrasion even when inks are layered. The important key to meeting this demand, was considered to reside in the constituents of the thermal-transfer recording medium, that is, the composition of the thermal-transfer ink layers in a so-called ink ribbon. Although the composition of transfer ink constituents has been studied conventionally, no proposal has been made yet which meets the level of the requirements of the invention. The object of the invention is to provide the method to solve this problem.